There are several questions and answers here about a potential advantage to launching from substantially higher altitudes, usually the answers are that while indeed there is some advantage, it is offset other questions of practicality such as infrastructure, transportation, and access to and proximity to water.

However, on Mars Olympus Mons provides an altitude advantage of 21 kilometers (something with no analog on Earth) and near Mars' equator at 19° North.

In this case, how much of an advantage would this be for a launch for a given vehicle? 1% in payload mass? 10%?

For balance, answers are welcome to point out the challenges and disadvantages as well.